Marvel Mystery Oil is an engine fuel and oil treatment... Helped worn engines run smoother back when 50,000 miles was about as old as an engine got without being re-built. Was most useful when all cars used carburetors especially before non-leaded gas became popular in the 70's. It was designed to clean out the jets in carburetors and to lubricate the top ring (compression ring) on the pistons. Jets have very precise tiny holes in then and if you try to use a piece of wire to clean it, you'll have to replace the jet...ask me how I know... Cleaning the jets and lubricating and getting a good seal on your compression ring gave the care more power, smoother running, and longer life, particularly on older cars.
It also helped remove carbon from the top of the top of the pistons and from the valves of the cylinders. Carbon build-up on top of the piston wasn't as big problem after unleaded gas became the standard but seemed to be a bigger problem with the exhaust valves. After that MMO still worked fine but didn't have as much to clean as it did with leaded gas use. You would sometimes get smoking out of your tailpipe right after you started using Marvel Mystery Oil as it burned off the carbon. If you had a car that would "diesel" (run on after you turned off the key). That was typically caused by carbon on the face of the pistons which would get hot and still ignite the gas even though there was no electrical spark to ignite it.
My youngest brother and I had a tune-up shop back in the 80's, and when we would encounter that dieseling problem, if the car hadn't been dieseling long,we would typically start the car, rev it up to about 2,000 rpm, and then slowly pour transmission fluid down the throat of the carburetor to clean that out. You had to make sure the car didn't stall out when you were doing it or it was really difficult to re-start. That process would smoke like you wouldn't believe, but it would clean the carbon out and stop the dieseling. Of course, if the carbon build-up was really bad, it could break off a chunk of carbon which could easily twist and damage the piston, cylinder, or get stuck in a valve. Marvel oil added to the gas would fix it without worrying about damaging anything. Just takes 2 or 3 tanks of gas to do it.
MOM added to the oil will lower the viscosity of the upper number by about 4 units but won't affect the bottom number. Upper number of 10-40 oil would drop to 10-36 but wouldn't change the low number (low number is for cold temps and high number is for hot temps). It adds an extra bit of lubrication without adding any metallics to it.
Although I've seen mentions of it as a penetrating oil, they make no mention of that on their website. It's still widely available in Auto Parts Shops and Lowes and Walmart both have it. Price is usually $5 or $6 there. Saw one price at $15 and another at $20 for larger size but $5-$6 bottle for the pint. They all used the same pictures but some were a gallon or more, which is probably more than a lifetime supply.
I did e-mail them to see if they claim it is a penetrating oil and we'll see what they say.
